Cleanup: Check for pthread in Libc

Some libc like Googles's Bionic for Android include pThread. Instead of
checking for Android as in commit
58a052effb2305655ff1b7233c78ab24fc25257e this patch checks for pthread
in libc as proposed by Thomas Petazzoni.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Charles Briere <charlesbriere.flatzo@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>